Malbim: I will gain that all residents of Egypt will know that I am Hashem. They will know that it was a punishment from Hashem, who is "Masgi ¹ la'Goyim va'Yabdem" (Iyov 12:23). After all this, Nebuchadnetzar came to Egypt and struck Egypt in its land. This happened several times. Here it explains the first war. It was a punishment for being a Mish'enes Kaneh to Yisrael. Yisrael relied on them, but they did not help. They only caused the Churban and downfall, for their reliance on Egypt removed their hearts from Hashem, and they did not heed Yirmeyah's words in the name of Hashem!

Rashi: Many times [Yisrael] relied on Egypt ? in the days of Sancheriv, in the days of Nebuchadnetzar, and Egypt did not help. This is like a reed, which is soft, and does not support one who leans on it.

Radak: They promised to save Yisrael from the Kasdim, and they could not. A reed is not strong. It supports only temporarily. It breaks easily, and all the more so when it is broken! Yonason translates 'support of a rickety stick.' The adjective (Re'i'a, i.e. rickety) is omitted, just like "ka'Tzemer Yihyu" (Yeshayah 1:18) means ka'Tzemer Lavan; "v'Yesh b'Edro Zachar" (Mal'achi 1:14) means a Zachar Tamim (without a Mum). Egypt was to Yisrael only a small support and help. "Pharaoh's army left Egypt, and the Kasdim besieging Yerushalayim heard, and departed from Yerushalayim. The Nevi'im told [Yisrael] 'Pharaoh, who came to help, returned to his land.' The Kasdim will return, fight against the city, conquer it and burn it" (Yirmeyah 37:5-8). Egypt did not support Yisrael ? they harmed them! Due to their reliance on Egypt, they rebelled against Melech Bavel! This is compared to a reed support.